The present invention relates to optical oscillators and, in particular, to improved means to either narrow the bandwidth of the optical oscillator or to tune the output of an optical oscillator, such as a dye laser.
Dye lasers have attracted much attention recently because of their property of being able to provide output wavelengths continuously tunable over comparatively large segments of the visible spectrum. In contrast, other lasers are only capable of providing strong output wavelengths at a limited number of discrete wavelengths.
In order to vary the output wavelength of a dye laser, suitable tuning apparatus must be provided. Presently, tuning is accomplished in a number of ways. One way is to place a prism within the optical cavity of the dye laser. As is well known, the prism refracts different wavelengths of light which pass through it at different angles. For laser oscillation to be maintained between the mirrors forming the optical resonator, the mirrors must be perfectly aligned so that reflected light is perpendicular to the surfaces of the optical resonator mirrors. Thus, by pivoting one of the optical resonator mirrors of the dye laser relative to the light passing through the prism, it is possible to allow laser oscillation only in a limited part of the spectrum.
Unfortunately, this technique has several major disadvantages. First, to obtain narrow linewidths, the resonator must be constructed so that it is extremely sensitive to angular misalignment, since the prism controls wavelength by angular dispersion. Second, angular misalignments between the pump laser and the dye laser result in variations in the dye laser output wavelength. Third, complicated means must be provided to insure that the position and direction of the output beam do not vary as the wavelength is varied. Fourth, to obtain narrow linewidths by using a prism, high dispersion glasses must be used, and these glasses introduce unwanted losses into the optical cavity and also exhibit thermal focusing which effectively changes the stability characteristics of the laser resonator.
A second approach to tuning a dye laser is the utilization of an adjustable optical grating within the optical resonator of the dye laser. By pivoting or rotating the grating, a desired output wavelength can be selected. This approach has the serious limitation of high optical losses.
A third approach includes the use of a lens having a relatively large longitudinal chromatic aberration at the output end of the dye laser. The wavelength of the light reflected from the output mirror and focused by the lens at the dye cell depends, because of the aberration, on the spacing of the lens away from the cell, and motion of the lens toward and away from the cell thus changes the wavelength of the light emitted by the laser. See, for example, U.S. Pat. No. 3,707,687. This arrangement also has disadvantages, mainly relatively poor wavelength control resulting in large bandwidths.
It is, therefore, an object of the invention to provide an improved technique for either narrowing the bandwidth of, or tuning, an optical oscillator.
Another object of the present invention is to provide an improved tuning apparatus for a dye laser.
Another object of the invention is to provide a tuning device for a dye laser which is capable of providing narrow bandwidth output from a dye laser over a broad range of the dye laser spectrum.
Another object of the invention is to provide an improved tuning means for a dye laser which is not overly sensitive and yet which is easy to construct and maintain in alignment.
In accordance with the present invention, a birefringent plate is placed within the optical cavity of an optical oscillator, such as a dye laser, at Brewster's angle to the light reflected therein. To alter the output wavelength of the optical oscillator, i.e. to tune or narrow its bandwidth, the birefringent plate is rotated about an axis which maintains the birefringent plate at Brewster's angle.
In accordance with another aspect of the invention, additional birefringent plates can be provided to further narrow the linewidth from the output of optical oscillator. Each of these additional birefringent plates is also positioned so that each is at Brewster's angle to the resonant optical mode within the optical cavity. As will be explained, the thickness of the birefringent plates must be related such that the thicker plates are an integral multiple of the thickness of the thinnest birefringent plate.